A. Field of the Invention
The present invention relates to an optical head adapted to read information from an optical disc by irradiating a laser light on the optical disc, and an optical recording and/or reproducing apparatus provided with the optical head.
B. Related Art
Referring now to FIG. 7, a conventional diffraction grating is shown. The diffraction grating is generally indicated by reference numeral 30. The diffraction grating 30 has formed therein a plurality of slits having a depth D and a cycle a. Assuming that the refractive index of a medium of the diffraction grating 30 upon which a light is incident is n0 and that of a medium of the diffraction grating 30 from which the light exits is n1, if the slit depth D is sufficiently smaller than the slit cycle a of the diffraction grating 30 (a >>D) and if the slit cycle a is sufficiently larger than the wavelength λ of a laser light incident upon the diffraction grating 30, the design of the diffraction grating will fit a scalar theory which will be described later. By a calculation based on the scalar theory fitting, the efficiency of diffraction of zero-order light of the diffraction grating 30, namely, a ratio in intensity between the incident light and zero-order light, is given by the following formula (1).cos2{(n1−n0)Dπ/λ}  (1) 
The efficiency of diffraction of first-order light of the diffraction grating 30 is given by the following formula (2):(2/π)2sin2{(n1−n0)Dπ/λ}  (2) 
For the conventional optical head provided with this diffraction grating 30 and adapted to split, by the diffraction grating 30, a laser light into three beams and effect a tracking servo control by the three-beam method and the conventional optical recording and/or reproducing apparatus using the optical head, the slit depth D of the diffraction grating 30 is selected for the formulae (1) and (2) to meet the following formula (3):0<(n1−n0)Dπ/λ<π/2  (3) 
In the optical head and optical recording and/or reproducing apparatus using a plurality of laser beams different in wavelength from each other, if the slit depth D of the diffraction grating 30 is selected based on the formula (3) in order to dispose the diffraction grating 30 in a common place where two laser beams of different wavelengths (λm and λn, and λm<λn) will pass, the efficiency of diffraction of first-order light of the laser beam having the wavelength λm is higher than that of the laser beam having the wavelength λn.
Then, when for a tracking servo control of the laser light of the wavelength λn, the light beam is split into three beams to adopt the three-beam method while for tracking servo control of the laser light of the wavelength λm, any method other than the three-beam method is adopted. More specifically, when it is desired to use the zero-order light, first-order light and negative first-order light of the laser light having the wavelength λn and only the zero-order light of the light beam having the wavelength λm, the laser light of the wavelength λm, is diffracted with a result that the efficiency for light power utilization is worse. At this time, undesired diffracted light is produced from the laser light of the wavelength λm, which leads to an increased tray light.